The present invention relates to methods and apparatuses for testing properties of materials, more particularly to methods and apparatuses for testing impact fracture toughness (resistance) of metallic materials such as titanium.
The xe2x80x9cdynamic tearxe2x80x9d (DT) test is a conventional procedure for characterizing impact fracture toughness properties of both ferrous and nonferrous materials. The xe2x80x9cone-inch standardxe2x80x9d DT test was developed at the United States Naval Research Laboratory (NRL) in the early 1960""s.
Since the mid-1960""s, the one-inch DT test has been used extensively by the U.S. Navy to characterize the impact fracture resistance of high strength steels, titanium alloys and aluminum alloys. Numerous U.S. Navy specifications require the use of the standard specimen for measuring the impact fracture toughness of candidate marine construction materials. DT test facilities have been established by the U.S. Navy at various research laboratories and production plants in this country and abroad.
Incorporated herein by reference, and appended hereto marked xe2x80x9cAPPENDIX A,xe2x80x9d is the following U.S. Navy report (NRL FR-6851) which describes the standard specimen and procedure in accordance with the one-inch DT test: Puzak, P. P. and F. A. Lange, xe2x80x9cStandard Method for the 1-inch Dynamic Tear Test,xe2x80x9d NRL Report 6851, February 1969. Essentially, as described by Puzak et al. in APPENDIX A, the standard one-inch DT test involves the impacting of a large beam which contains a brittle crack starter weld on the tension side of the standard specimen. The dimensions of the standard specimen are 1 inch thickxc3x974.75 inches widexc3x9718 inches long.
Also incorporated herein by reference are the following U.S. Navy reports: NRL FR-7159, E. A. Lange, P. P. Puzak and L. A. Cooley, xe2x80x9cStandard Method for the ⅝ Inch Dynamic Tear Test,xe2x80x9d 1970; NRL FR-6975, E. A. Lange and F. J. Loss, xe2x80x9cDynamic Tear Energyxe2x80x94A Practical Performance Criterion for Fracture Resistance,xe2x80x9d 1969; NRL FR-6991, C. N. Freed and R. J. Goode, xe2x80x9cRelationship between Fracture Toughness and Estimated Plastic Zone Size in Steel, Titanium and Aluminum Alloys,xe2x80x9d 1969; NRL FR-6873, R. W. Judy, P. P. Puzak and E. A. Lange, xe2x80x9cCharacterization of Fracture Toughness of 5Nixe2x80x94Crxe2x80x94Moxe2x80x94V Steel by Charpy V-Notch and Dynamic Tear Tests,xe2x80x9d 1969; NRL FR-6864, G. E. Nash, xe2x80x9cAn Analysis of the Forces and Bending Moments Generated during the Dynamic Tear Test,xe2x80x9d 1969; R-1969-14151, G. E. Nash, E. A. Lange, xe2x80x9cMechanical Aspects of the Dynamic Tear Test,xe2x80x9d 1969; NRL MR-1826, G. E. Nash and E. A. Lange, xe2x80x9cMechanical Aspects of the Dynamic Tear Test Specimen,xe2x80x9d 1967; R-1970-14063, E. A. Lange and F. J. Loss, xe2x80x9cDynamic Tear Energyxe2x80x94A Practical Peformance Criterion for Fracture Resistance,xe2x80x9d 1970; NRL FR-7056, F. J. Loss, xe2x80x9cDynamic Tear Test Investigations of the Fracture Toughness of Thick-Section Steel,xe2x80x9d 1970; NRL FR-6993, L. A. Cooley and E. A. Lange, xe2x80x9cVertical Drop-Weight Machine for Conducting Drop-Weight NDT, Drop-Weight Tear and Dynamic Tear Tests,xe2x80x9d 1970; R-1977-14178, T. G. Heberling, E. S. Harris and E. A. Lange, xe2x80x9cResults of Interlaboratory Test Programs to Evaluate the ⅝-in. (16-mm) Dynamic Tear Test Method,xe2x80x9d 1977.
According to the one-inch standard DT test, the brittle crack starter weld is prepared by machining a shallow 1.75 inch long groove across the width on each of both sides of the specimen. A small amount of embrittling material is diffused in an electron beam weld through the 1-inch plate thickness. For a titanium specimen, steel wires are placed in the grooves and upset by light hammering to ensure uniform distribution of the embrittling materials during electron beam (EB) welding. After EB welding the crack starter weld is notched to assist initiation of the crack in the embrittled weld. The specimen is fractured using a pendulum machine (or drop-weight machine), and the total energy for fracture is recorded.
The main disadvantage associated with use of the standard one-inch DT test (to characterize the fracture toughness of materials) is the requisite large size of the test specimen. Although usually one-inch DT testing of flat plate material is readily accomplished, one-inch DT testing of many different product and pre-product forms, such as used in submarine and surface ship construction, are problematical or impractical.
For instance, forgings and castings are frequently of a shape that does not allow the removal of an 18-inch long flat specimen. This necessitates the design and fabrication of special forging dies that produce a sufficient amount of excess material to allow the removal of a DT specimen blank for testing. Another example of the limitedness of the one-inch DT test is its inability to test small mock-up or pre-production forgings, which typically are on the order of 9-12 inches in diameter. The cost of fabricating special forging dies for mock-up or pre-production testing can be prohibitively expensive.
In view of the foregoing, it is an object of the present invention to provide method and apparatus for testing dynamic tear of a metallic material when there is an insufficient and/or unsuitably dimensioned amount of available metallic material for properly performing the one-inch standard DT test.
It is further object of the present invention to provide method and apparatus for testing dynamic tear of a limited amount of metallic material so that one or more other tests of the metallic material can be properly performed.
The present invention provides a methodology for measuring the dynamic tear toughness properties of metallic materials (such as titanium, steel and aluminum alloys), particularly in cases of limited material availability or incompatible size and geometry. The inventive DT test procedure features utilization of a modified (xe2x80x9cnonstandardxe2x80x9d) DT specimen in lieu of the xe2x80x9cstandardxe2x80x9d one-inch DT specimen. Inventively implemented is a smaller, xe2x80x9cnonstandardxe2x80x9d DT specimen which will generate energy values equivalent to those which would be measured for the larger, xe2x80x9cstandardxe2x80x9d one-inch DT specimen.
According to this invention, the nonstandard one-inch dynamic tear specimen comprises a center (middle) test section and a pair of end-tabs. The center test section is made of the test material, and is joined (preferably, welded) to each of the two end-tabs. The two end-tabs, when appropriately combined with the center test section, accomplish complete dimensional standardization of the nonstandard one-inch DT specimen.
In other words, according to this invention, the center test section and the two end-tabs each have the standard width and standard thickness, but a xe2x80x9cnon-standardxe2x80x9d length which is something less than the standard length. In typical inventive practice, the two end-tabs will each have the same xe2x80x9cnon-standardxe2x80x9d length, which will exceed the xe2x80x9cnon-standardxe2x80x9d length of the center test section. Once the center test section and two end-tabs are united in the lengthwise direction, the integral whole has become characterized by the standard length as well as the standard width and standard thickness.
In accordance with the present invention, a method is provided for obtaining a rectangular parallelepiped section from an object for the purpose of being subjected to dynamic tear testing of the kind wherein a rectangular parallelepiped specimen made of a metallic material is impacted, wherein the specimen has a prescribed length, a prescribed width and a prescribed thickness, and wherein the specimen has provided therein a crack through the thickness and a portion of the width for initiating fracture of the specimen when impacted. The method comprises: determining the extent of lengthwise plastic deformation of the specimen which would result if the specimen were subjected to the dynamic tear testing; and, obtaining from the object the rectangular parallelepiped section having the prescribed width, the prescribed thickness and a nonprescribed length which is shorter than the prescribed length, the nonprescribed length being at least as great as the determined extent of lengthwise plastic deformation.
Further provided in accordance with the present invention is a method for making a rectangular parallepiped integral entity to be used for the purpose of performing, in relation to an object, dynamic tear testing of the kind wherein a rectangular parallepiped specimen made of a metallic material is impacted, the specimen having a prescribed length, a prescribed width and a prescribed thickness, the specimen having provided therein a crack through the thickness and along a portion of the width for initiating fracture of the specimen when impacted. The method comprises: determining the extent of lengthwise plastic deformation of the specimen which would result if the specimen were subjected to the dynamic tear testing; obtaining from the object a rectangular parallelepiped section having the prescribed width, the prescribed thickness and a section length which is shorter than the prescribed length, the section length being at least as great as the determined extent of lengthwise plastic deformation; and, joining at lengthwise opposite ends of the section a pair of congruent rectangular parallelepiped end-tabs, each end-tab having the prescribed width, the prescribed thickness and an end-tab length which is equal to one-half the difference between the prescribed length and the section length, thereby forming the integral entity having the prescribed length, the prescribed width and the prescribed thickness, the integral entity being adaptable to being used as the specimen for the purpose of performing the dynamic tear testing.
Still further provided by the present invention is a method for performing, in relation to an object, dynamic tear testing of the kind wherein a rectangular parallepiped specimen made of a metallic material is impacted, the specimen having a prescribed length, a prescribed width and a prescribed thickness, the specimen having provided therein a crack through the thickness and along a portion of the width for initiating fracture of the specimen when an impact is caused with respect thereto. The method comprises: determining the extent of lengthwise plastic deformation of the specimen which would result if the specimen were subjected to the dynamic tear testing; obtaining from the object a rectangular parallelepiped section having the prescribed width, the prescribed thickness and a section length which is shorter than the prescribed length, the section length being at least as great as the determined extent of lengthwise plastic deformation; joining at lengthwise opposite ends of the section a pair of congruent rectangular parallelepiped end-tabs, each end-tab having the prescribed width, the prescribed thickness and an end-tab length which is equal to one-half the difference between the prescribed length and the section length, thereby forming the integral entity having the prescribed length, the prescribed width and the prescribed thickness, the integral entity being adaptable to being used as the specimen for performing the dynamic tear testing; providing the crack in said integral entity; and, causing the impact with respect to the integral entity.
In inventive practice, it is not necessary for either end-tab that the material composition of the end-tab match the material composition of the center test section. In fact, it is not even inventively necessary that the end-tabs match each other in terms of material composition. It is inventively necessary, however, that each end-tab have such material composition as to be capable of being welded to the center test section.
As for xe2x80x9cweldabilityxe2x80x9d of the end-tabs with respect to the center test section, the end-tabs should be made of a compatible material for such purposes. Metallic materials are broadly classified as either ferrous or non-ferrous. The inventive practitioner should use ferrous end tabs with a ferrous center test section, and should use non-ferrous end tabs with a non-ferrous center test section. Beyond that, inventive practice will generally require that each end tab""s material composition belong to the same metallic material grouping as does the center test section""s material composition. The U.S. Navy and some other entities use a classification system which provides for five groups or categories of metallic materials, viz., (i) steel, (ii) aluminum and aluminum alloys, (iii) copper and copper alloys, (iv) titanium and titanium alloys, and (v) nickel and nickel alloys. Of these five metallic material categories, steel is ferrous; aluminum/aluminum alloys, copper/copper alloys, titanium/titanium alloys and nickel/nickel alloys are non-ferrous.
In inventive practice, it is generally requisite that the center test section and the two end-tabs all be made of a material which belongs to the same metallic material category. More specifically, a steel center test section should be used with two steel end-tabs. An aluminum or aluminum alloy center test section should be used with two aluminum or aluminum alloy end-tabs. A copper or copper alloy center test section should be used with two copper or copper alloy end-tabs. A titanium or titanium alloy center test section should be used with two titanium or titanium alloy end-tabs. A nickel or nickel alloy center test section should be used with two nickel or nickel end-tabs. Violation of this inventive xe2x80x9csame-groupxe2x80x9d principle will likely compromise the inventive testing.
For instance, titanium is highly reactive with practically all materials. Titanium alloy material reacts with virtually any non-titanium alloy material to form brittle inter-metallic compounds (e.g., in the weld). If, for example, a titanium alloy center test section were used with steel end-tabs, the welds joining the center test section to the end-tabs would be embrittled and hence would compromise the performance of the inventive nonstandard specimen.
According to the present invention, the center section could be made of a first material (e.g., a first titanium alloy), and the end-tabs could each be made of a second material (e.g., a second titanium alloy). Alternatively, the center section could be made of a first material (e.g., a first titanium alloy), one end-tab could be made of a second material (e.g., a second titanium alloy), and the other end-tab could be made of a third material (e.g., a third titanium alloy). Again alternatively, the center section and one end-tab could each be made of a first material (e.g., a first titanium alloy), and the other end-tab could be made of a second material (e.g., a second titanium alloy). Further alternatively, the center section and the end-tabs could each be made of a first material (e.g., a first titanium alloy); that is to say, the center section and end-tabs could all be made of the same material (e.g., the same titanium alloy). An inventive advantage of using at least one end-tab having a different material composition from that of the center section, albeit within the same material grouping, is that such different material composition is readily available at significantly less cost.
The end-tabs are joined to the center test section by means of electron beam welding. The inventive procedures pertaining to (i) electron beam welding of the crack starter, and (ii) notching of the crack starter, are similar to the corresponding procedures used for the standard one-inch dynamic tear specimen.
According to many inventive embodiments involving one-inch dynamic tear testing, the center test section is about five inches wide, and each end-tab is about 6xc2xd inches long. Hence, the test material of the inventive nonstandard DT specimen is approximately 5 inches in length, compared with approximately 18 lengthwise inches of test material on the standard 1-inch DT specimen.
The five inch length of the center test section (for one-inch DT testing) does not represent an arbitrary selection by the inventors; rather, the inventors determined that all of the plastic deformation for the titanium alloys tested was contained within this center section. In generally preferred inventive practice pertaining to one-inch DT testing, the center test section is at least approximately 5 inches long, and (depending on the inventive embodiment) can be shorter than 5 inches.
Five inches is the longest possible center test section length which is reasonably expected to encompass the entirety of plastic deformation, when subjected to one-inch DT testing. Just five inches is an expeditious center test section length for many inventive embodiments involving one-inch DT testing, since the requisite amount of test material is minimized while being fully inclusive of plastic deformation.
The inventive practitioner will be capable of determining the extent of plastic deformation of a particular type of metallic material. This determination will govern the length of the inventive center test section in the dynamic tear testing. This determination is easily accomplished by undertaking xe2x80x9cbeforexe2x80x9d and xe2x80x9cafterxe2x80x9d measurements (e.g., using a micrometer) of a standard 18-inch long DT test specimen. First, the width and thickness of the specimen are measured. Then the specimen is subjected to standard 1-inch dynamic tear testing. Then, the specimen is measured again. The practitioner measures the change in thickness from the fracture edge outward (i.e., change in thickness while moving bidirectionally away from the fracture edge). The thickness along the fracture plane (face) is measured by commencing at the center of the specimen and proceeding in opposite directions. The starting point is the center of the specimen. For instance, if the width of the specimen is 4xc2xe inches, the practitioner commences measurement at 2xe2x85x9c inches and proceeds in either direction. The plastic deformation has ceased where the post-test thickness is equal to the pre-test thickness.
U.S. Navy researchers have demonstrated the feasibility of inventive practice using a nonstandard specimen made of the near alpha titanium alloy Ti 6Alxe2x80x942Cbxe2x80x941Taxe2x80x941Mo and of the alpha-beta titanium alloy Ti 6Alxe2x80x944V. Incorporated herein by reference, and appended hereto marked xe2x80x9cAPPENDIX B,xe2x80x9d is the following U.S. Navy report: Wells, Michael E. and Roy A. Lindauer, xe2x80x9cA Test Method for Dynamic Tear Testing of Titanium Plate Using a Nonstandard Specimen,xe2x80x9d CARDIVNSWC-TR-61-97/11, August 1998, Technical Report, Survivability, Structures and Materials Directorate, Carderock Division, Naval Surface Warfare Center, West Bethesda, Md. 20817-5700.
APPENDIX B discloses how U.S. Navy researches inventively characterized the fracture toughness properties of an alpha-beta titanium alloy. The present invention is described therein in terms of the dimensions and preparation of the specimen, the apparatus and the testing procedure. As disclosed by Wells et al. in APPENDIX B, the inventive nonstandard specimen proved adequate for measuring the fracture energy of titanium alloy Ti 6Alxe2x80x944V plate. The results of the inventive dynamic tear tests demonstrated that the inventive nonstandard specimen accurately measures the fracture energy of titanium alloy plate.
It is believed, however, that the present invention can be applied to all high strength titanium, steel and aluminum alloys, and to some other metallic materials. Especially in cases of limited or incompatible material availability, the inventive methodology is recommended for characterization of the fracture toughness properties not only of alpha-beta titanium alloys, but of a variety of other metallic materials as well.
As noted hereinabove, titanium alloys Ti 6211 and Ti 6Alxe2x80x944V were used by the U.S. Navy to demonstrate that the inventive nonstandard DT specimen could be used in lieu of the standard one-inch DT specimen to accurately measure the fracture toughness of near alpha (Ti 6211) and alpha-beta (Ti 6-4) alloys. Nevertheless, since the amount of plastic deformation decreases with increasing strength, the inventors believe it manifest that the nonstandard specimen can be used to test all higher strength titanium alloys.
A preliminary examination of the plastic zone in low strength, commercially pure titanium nonstandard specimens revealed that the plastic deformation did not appear to extend beyond five inches. Therefore, in inventive practice, the maximum length of the center test section is five inches, which will encompass the plastic deformation in all titanium alloys. It is believed likely that this specimen""s dimensional configuration can also be applied to all titanium alloys and to steel and aluminum alloys (and other metallic materials) of comparable strength.
In accordance with the present invention, the center test section should have a length which ensures complete envelopment of the plastic deformation which it shall experience upon undergoing dynamic tear testing. As a general xe2x80x9crule of thumbxe2x80x9d for inventive practice involving one-inch dynamic tear testing, five inches is the maximum length which, regardless of the material of the center test section, will guarantee inclusion of its plastic deformation when it is subjected to the one-inch dynamic tear testing. However, some metallic materials lend themselves to inventive dynamic tear testing using a center test section which is less, perhaps much less, than five inches long.
In this regard, specific inventive guidelines for selecting center test section length can be inventively ascertained for different types of materials in accordance with subcategorizations thereof. It is recalled that five categories of metallic materials are conventionally recognized, viz., (i) steel, (ii) aluminum and aluminum alloys, (iii) copper and copper alloys, (iv) titanium and titanium alloys, and (v) nickel and nickel alloys. For instance, the xe2x80x9ctitanium and titanium alloyxe2x80x9d metallic material category is conventionally grouped into four groups, designations or subcategories, viz., (i) the unalloyed or commercially pure subcategory, (ii) the alpha and near alpha subcategory, (iii) the alpha-beta subcategory, and (iv) the beta subcategory. It has been inventively determined that, in relation to one-inch DT testing, the minimum center test section lengths corresponding to these four subcategories of the titanium/titanium alloy category are as follows: (i) five inches for the unalloyed or commercially pure subcategory, (ii) four inches for the alpha and near alpha subcategory, (iii) three inches for the alpha-beta subcategory, and (iv) two inches for the beta subcategory.
The minimum center test section lengths corresponding to any or all of the four subcategories of the titanium/titanium alloy category can similarly be inventively determined in relation to other-dimensioned (e.g., ⅝ inch) dynamic tear testing. Moreover, the minimum center test section lengths corresponding to any or all of the designations or subcategories of any or all of the remaining metallic material categories (namely, steel, aluminum/aluminum alloys, copper/copper alloys and nickel/nickel alloys), and in relation to any or all of types of dynamic tear testing (particularly recognized types of DT testing, e.g., 1-inch DT testing, xe2x85x9c-inch DT testing, etc.) can also be determined in accordance with the present invention.
Notable among the advantages of the inventive nonstandard one-inch DT specimen are the present invention""s ability to measure the fracture toughness of small mock-ups or pre-production forgings. Also notable is this invention""s ability to measure the fracture toughness of finished forgings without incurring the additional expense of fabricating special forging dies.
Conventionally, for purposes of conducting fracture toughness testing, a special forging die is designed and fabricated to produce excess material from the finished forging. Typically, this excess material is machined from the finish forging and then subjected to fracture toughness tests. It has been the experience of the U.S. Navy that these prolongations may not accurately represent all of the working experienced by the finished product. By comparison, the relatively small test section size of the inventive nonstandard specimen beneficially allows for the testing of the actual forged part.
Mock-ups or pre-production forgings, castings and extrusions are used extensively to screen the effects of different forging/temperature combinations on mechanical properties. The typical small size of the pre-production product precludes the use of the standard 1-inch DT specimen. With inventive acceptance criteria being based on those associated with the standard one-inch DT test, the inventive nonstandard 1-inch DT specimen can be used to accurately gage the effects of various processing combinations on fracture toughness prior to production forging.
Another inventive advantage relates to material savings. The standard one-inch DT specimen is materially intensive. The amount of material required to produce a single standard DT specimen can be used to produce three inventive nonstandard DT specimensxe2x80x94and with some material left over.
Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
The following appendices are hereby made a part of this disclosure:
Attached hereto marked APPENDIX xe2x80x9cAxe2x80x9d and incorporated herein by reference is the following aforementioned 22-page U.S. Navy report which discloses various aspects of the standard one-inch DT test: Puzak, P. P. and F. A. Lange, xe2x80x9cStandard Method for the 1-inch Dynamic Tear Test,xe2x80x9d NRL Report 6851, February 1969.
Attached hereto marked APPENDIX xe2x80x9cBxe2x80x9d and incorporated herein by reference is the following eighteen-page U.S. Navy report which discloses various aspects of the inventive nonstandard one-inch DT test: Wells, Michael E. and Roy A. Lindauer, xe2x80x9cA Test Method for Dynamic Tear Testing of Titanium Plate Using a Nonstandard Specimen,xe2x80x9d CARDIVNSWC-TR-61-97/11, August 1998, Technical Report, Survivability, Structures and Materials Directorate, Carderock Division, Naval Surface Warfare Center, West Bethesda, Md. 20817-5700.